Color improvement of phosphate esters

ABSTRACT

A process for the permanent removal of color from phosphate esters of alkylphenols which either are colored or become colored on storage by simultaneously bleaching and color-stabilizing the phosphate esters with sodium borohydride or lithium borohydride.

ilnite fittes Patent Patelet al.

[ 51 Aug..1,1972

[54] COLOR IRQVENT 0F FHOSPTE ESTERS [72] Inventors: Vitlml C. Patel,Glen Burnie; Ed-

ward F. Orwoll, Baltimore, both of [73] Assignee: FMC Corpus-alien, NewYork, NY.

{22] Filed: Feb. 4, 1970 [21] Appl. No.: 8,689

[52] US. Cl. ..260/989, 260/966, 260/990 [51] Int. Cl. ..C07E 9/08, C07f9/12 [58] Field of Search ..260/989, 990, 966, 967

[56] References Cited UNITED STATES PATENTS 3,356,775 12/1967 Mitchell..2 60/990 3,1'50151 9/1964 Oldenburg ..260/621 A 3,202,719 8/1965 Jones..260/966X Primary Examiner-Joseph Rebold Assistant Examiner-Richard L.Raymond Attorney-Eugene G. Seems, Charles C. Fellows and Pauline Newman5 7] ABSTRACT 6 Claims, No Drawings COLOR IMPROVEMENT OF PHOSPHATEESTERS BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention relates to the preparation of low-color and color-stablealkylphenol esters of phosphoric acid.

2. Description of the Prior Art The production of low-color andcolor-stable phosphate esters made from alkylphenols has long beenrecognized to be a problem. Moreover, it has been noted that as thealkyl substitution of the aryl ring increases in going from tricresylphosphate, i.e., tris(methylphenyl) phosphate, to compositions withincreasing alkyl substitution, for example, tris(dimethylphenyl)phosphate, color increases. For example, tricresyl phosphate is sold asa commercial plasticizer typically having an acceptable platinumcobaltcolor of 75, as determined according to ASTM D-1209-2. Another triarylphosphate ester, that is made from mixed cresols and xylenols and havingabout equal numbers of cresol and xylenol aryl groups, has a maximumplatinum-cobalt color of 100. A triaryl phosphate made from a mixture of2 k of xylenol to each half part of cresol, which is commerciallyavailable, typically has a platinum-cobalt colorof 200 and the so-calledtrixylyl phosphate has a platinum-cobalt color of 250.

A number of patents have issued on various methods for producinglow-color, color-stable phosphate esters made from alkylphenols. Scott,in U.S. Pat. No. 1,958,210, issued May 8, 1934, discloses the use ofdecolorizing carbon for reducing the color of phosphate esters. Shumanin U.S. Pat. No. 2,113,951, issued Apr. 12, 1938, discloses a method formaking color-stable phosphate esters by a route which involvesdistilling a phenol in the presence of a mineral acid such as sulfuricacid, hydrochloric acid, phosphoric acid, etc. to purify it, andemploying the purified phenol to make tricresyl phosphate esters whichare more stable to the action of heat and sunlight than esters made fromphenols distilled in the absence of an inorganic acid.

A method of phenol color reduction is disclosed by Oldenburg in U.S.Pat. No. 3,150,191 to overcome a particular color problem in the case ofphenols produced by the so-called cumene process. Cumeneprocess phenols,when chlorinated, produce undesirable color formation. However, bycontacting the phenols with borohydride ions prior to chlorinating,chlorophenols of low color are produced.

Recently, a shortage of cresols and other suitable feed stocks for usein producing triaryl phosphate esters has made it desirable to makesynthetic alkylphenols. Typical synthetic alkylphenols are mixedisopropylphenols. These mixed isopropylphenols have a good color, andwhen they are reacted with phosphorus oxychloride to make thecorresponding phosphate ester, the ester itself can be made with goodinitial color, although frequently the ester has a platinum-cobalt colorof 300-500. Surprisingly, although even when they have a good initialcolor, phosphate esters made from mixed isopropylphenols when stored inthe dark change from a platinum-cobalt color of about 50-60 to a lemonyellow color of 300 to 500.

A process for decolorizing and color-stabilizing phosphate esters madefrom alkylphenols, and particularly isopropylphenols, against colorformation is a principal object of this invention.

SUMMARY OF THE INVENTION We have now discovered a process for thepermanent removal of color from tri(alkylphenyl) phosphate esters havingexcessive yellow color by bleaching the phosphate esters with 0.002 to0.05 percent by weight of an alkali metal borohydride selected from thegroup consisting of sodium borohydride and lithium borohydride. Theselected borohydride may be added either as a solid, or as a liquidsolution of 0.01 to 10 percent by weight concentration. When phosphateesters are bleached by being contacted with 0.002 to 0.05 percent byweight based on the weight of the phosphate ester of a borohydrideselected from the group consisting of sodium borohydride and lithiumborohydride, and the resulting mixture is stirred from 5 to 24 hours,the phosphate ester not only is bleached, but surprisingly is stabilizedagainst subsequent color formation.

DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENTS Alkylphenylphosphate esters are typically made by reacting a cresol or alkylatedphenol and phosphorus oxychloride at a molar ration of 3:1 in a reactionvessel equipped with a reflux condenser, by raising the temperature toabout 180 C. or higher until the evolution of hydrogen chloride gasceases. The resulting crude triaryl phosphate is distilled under vacuumand washed with a dilute alkali solution having a concentration of about2 percent to remove free acids and unreacted alkylphenols. The productis frequently further processed to remove traces of color or odor bywashing with water-soluble permanganate solutions or by treating withdecolorizing agents such as activated carbon.

Permanent removal of color from discolored alkylphenyl esters accordingto the method of this invention is achieved by treating the phosphateester with sodium borohydride or lithium borohydride. The preferredborohydride is sodium borohydride because it is the least costly of theborohydride materials commercially available. Sodium borohydride andlithium borohydride are used in effective amounts of 0.002 to 0.05percent by weight based on the weight of the phosphate ester beingbleached and stabilized. Levels higher than 0.05 percent may be used buthave no added advantage and are uneconomical. The preferred amount isabout 0.01 to 0.02 percent by weight. Surprisingly, the closely relatedpotassium borohydride is not very effective as a bleaching andcolor-stabilizing agent for tri( alkylphenyl) phosphate esters.

We bleach and color-stabilize tri(alkylphenyl) phosphates by stirringthem together with solid borohydride for from 5 minutes to 24 hours.Generally, the ester is treated for about 5 minutes to 1 hour at atemperature of about 20 to C., although preferably the treatment iscarried out at about 45 to 55 C. The hot treatment can be followed by upto 24 hours stirring at ambient temperature. A moderate temperature ofabout 50 C. is preferred as the higher temperatures tend to decomposethe sodium borohydride without properly bleaching the phosphate ester.The bleach can be carried out at higher than atmospheric pressure,although atmospheric pressure is preferred as a matter of convenience.The phosphate ester may be diluted with an inert organic solvent and theborohydride can be used in solution in a solvent if desired, although itis preferred not to use any kind of a solvent or diluent for either thephosphate ester or the borohydride.

The phosphate ester can be treated with solid borohydride or an aqueoussolution of borohydride. The solid treatment is preferable because ofthe ease of work-up after the treatment. The borohydride treatment of aphosphate ester can be carried out batchwise or in a continuous manner.

The borohydride treatment should be done under neutral or alkalineconditions, as strongly acidic conditions decompose sodium borohydrideinstantly. The presence of moisture in the phosphate ester does notadversely affect the bleaching properties of sodium or lithiumborohydride.

The effective concentration of sodium or lithium borohydride depends onthe history of the phosphate ester. Excessive exposure of tri(alkylphenyl) phosphate esters to air during purification causes abuild-up of color which is difficult to remove, and as much as 0.05percent sodium borohydride may be required to bleach the ester andstabilize it against subsequent color formation. Phosphate esters madeunder normal processing conditions generally require 0.01 percent orless by weight of solid sodium borohydride to effect adequate bleachingand to stabilize the ester against subsequent color formation. The timerequired to bleach a phosphate ester also depends upon its historyduring its production. Phosphate esters which have undergone excessiveexposure to air during purification develop a stubborn color thatresists bleaching, and as long as 24 hours may be required to obtain anadequate bleach. However, in most cases involving tri(alkylphenyl)phosphate esters produced under normal operating conditions, itgenerally takes about 1 hour with a 0.01 percent sodium borohydrideconcentration to effect bleaching and color stabilization.

The rate of bleaching, aS well as the rate of decomposition of theborohydride, increases as the temperature increases. The bleach can beconducted at temperatures between 20 and about 100 C. However, practicehas taught that about 50 C. is a good choice of an average temperatureat which to bleach the phosphate esters. Good agitation, suflicient tokeep the borohydride uniformly suspended in the phosphate ester duringbleaching, is necessary. At the end of the bleaching period, theunreacted borohydride and byproducts are filtered from the phosphateesters by ordinary filtration procedures.

The following examples, illustrating the novel method disclosed herein,are given without any intention that the invention be limited thereto.All parts and percentages are by weight, unless otherwise noted.

EXAMPLE 1 A 500 gram sample of a tri(alkylphenyl) phosphate with aplatinum-cobalt color of 200, determined according to ASTM D-1209Standard Method Test for Color of Clear Liquids (platinum-cobalt scale),made from mixed isopropylphenols composed of a mixture ofmonoisopropylphenols, diisopropylphenols, triisopropylphenols and someunsubstituted phenol, was washed with a 2 percent by weight solution ofsodium hydroxide in water and then treated with carbon at 120 C. To thedried phosphate ester was added 0.05 grams of sodium borohydride, andthe mixture was stirred in the dark at 50 C. for 30 minutes, after whichthe mixture was filtered warm to remove the unreacted sodium borohydrideand solid by-products. The treated ester had a platinum-cobalt colorrating of 25. Samples of the untreated and bleached product were storedin the dark for 12 months during which the untreated sample had turnedyellow while the color of the treated sample remained unchanged at about25 platinumcobalt color.

EXAMPLE 2 Example 1 was repeated using 0.1 grams of sodium borohydrideto treat 500 grams of the tri(alkylphenyl) phosphate. The color of theproduct was reduced to a platinum-cobalt color of 20.

EXAMPLE 3 A 500 gram sample of tri(propylphenyl) phosphate was treatedas described in Example I with 0.05 grams of sodium borohydride and theplatinum-cobalt color of the phosphate was reduced from to 10.

EXAMPLE 4 Example 1 was repeated using only 0.025 grams of sodiumborohydride to treat 500 grams of the tri(alkylphenyl) phosphate. Theplatinum-cobalt color of the tri(alkylphenyl) phosphate was reduced from300 to 40.

EXAMPLE 5 To 27.5 pounds of tri(propylphenyl) phosphate was added0.00275 pounds (0.01 percent) of sodium borohydride and the resultingmixture was stirred in the dark at 50 C. for 1 hour. The mixture wasfiltered warm. The platinum-cobalt color of the tri(propylphenyl)phosphate was reduced from 300 to 15.

EXAMPLE 6 Example 1 was repeated using a tri(alkylphenyl) phosphate withan initial platinum-cobalt color of 100. After addition of the sodiumborohydride, the temperature of the phosphate-borohydride was raised to75 C. and the temperature of the mixture was maintained at 75 C. for 15minutes. The tri(alkyl-phenyl) phosphate, after treatment, had aplatinum-cobalt color of 25.

EXAMPLE 7 Example 6, was repeated except that the phosphate ester waspreheated to 75 C., sodium borohydride was added, and the mixture wasmaintained at 75 C. for 15 minutes. The platinum-cobalt color of thephosphate ester was reduced from to 50.

EXAMPLE 8 Example 7, was repeated using a treating temperature of 100 C.and 0.02 gram of sodium borohydride was used. The platinum-cobalt colorof the phosphate ester was reduced from 100 to 60.

EXAMPLE 9 A 100 gram sample of the tri(alkylphenyl) phosphate of Example1 was washed with a 2 percent by weight solution of sodium hydroxide inwater and the washed sample was then treated with 0.5 parts by weight ofactivated carbon at 120 C. and filtered. The filtered phosphate wastreated with 10 grams of a 0.1 percent 1 by weight solution of sodiumborohydride in water, and the aqueous layer which formed was separatedfrom the phosphate ester by decantation. The treated ester was thendried by heating at ll0-l30 C. for minutes under nitrogen. Theplatinum-cobalt color of the phosphate ester was reduced from 300 to 20.

EXAMPLE 10 A 200 gram sample of the tri( propylphenyl) phosphate wasmixed with 0.02 gram of lithium borohydride and stirred in the darkwhile the temperature was being raised to 50 C. In less than 8 minutesafter adding the lithium borohydride and before the temperature reached50 C., the color essentially disappeared. The mixture was filtered warm.The platinumcobalt color of the phosphate ester was reduced from 400 to10 by the treatment.

COMPARISON EXAMPLE A To a 200 gram sample of the phosphate ester ofExample 10 was added 0.10 gram of potassium borohydride. The mixture wasstirred in the dark at 50 C. for 4 hours with a very slight reduction inplatinumcobalt color of 400 to about 370. No more reduction in color wasobserved, even after 24 hours treatment at room temperature. Thisexample was repeated using an aqueous solution of the potassiumborohydride and again there was practically no reduction in color.

COMPARISON EXAMPLE B Tris(isopropylphenyl) Phosphate Made fromlsopropylphenols Bleached with Sodium Borohydride A mixture of 650 gramsof synthetic isopropylphenols, as described in Example 1, but darker incolor (Gardner scale color-9)*, 325 grams of distilled water, and 1.95grams of sodium borohydride was stirred at 50 C. for 2 hours. Theresulting mixture was distilled at atmospheric pressure to obtain 602grams of practically water-white (platinum-cobalt color- 10)isopropylphenols.

A one-liter, three-neck round-bottom flask,

equipped with condenser, thermometer, stirrer and addition funnel, wascharged with 600 grams of distilled isopropylphenols (4.61 moles ofmixed phenols) and 3.0 grams (0.025 mole) of aluminum chloride catalyst.To the reaction mixture, at 1l5l20 C., was added 196 grams 1.28 mole) ofphosphorus oxychloride over a 2-hour period. The resulting mixture wasstirred at 0 180 C. for 3 hours. It was then allowed to cool to aboutThis invention has been explained and exemplified in I a manner so thatit can be readily practiced by those skilled in the art. The best modecontemplated by the inventor has been set forth. Clearly, within thescope of the appended claims, the invention can be practiced by thoseskilled in the art having the benefit of the disclosure, otherwise thanas specifically described and exemplified herein.

What is claimed is:

1. A process for bleaching and color-stabilizing a tri(alkylphenyl)phosphate ester comprising bleaching the ester with from 0.002 percentto 0.05 percent by weight based on the phosphate ester of an alkalimetal borohydride selected from the group consisting of sodiumborohydride and lithium borohydride by mixing the borohydride togetherwith the ester at a temperature between 20 and C. for a period of from 5minutes to 24 hours.

' 2. The process of claim 1 in which the alkali metal borohydride issodium borohydride.

3. The process of claim 1 in which the alkali metal borohydride islithium borohydride.

4. The process of claim 1 in which the temperature is between 45 and 55C.

5. The process of claim 1 in which the processing period is between 15minutes and 1 hour.

6. The process of claim 1 in which the alkali metal borohydride ispresent in an amount of 0.01 to 0.02 percent by weight based on thephosphate ester.

UNTTED STATES PATENT @FFTCE QERTTTTQATE CT CCRREQTTCN Patent No. 2 68 1L[82 Dated August 1, 1972 Inventor(s) Vithal C. Patel and Eward F.Orwoll It is certified that error appears in the above-identified patentand that said Letters Patent are hereby corrected as shown below:

Column 1, line 20, "2" should read---6 2--.

Column 2, line 30, "ration" should read --ratio--,.

Column 3, line U6, "aS" should read -as-.

Column l, line 55, "tri(alkylphenyl) should read tri(alkylphenyl)-.

Column 5, line &6, The following footnote relating to the asterisk wasomitted:

*Gardner scale color, determined according to ASTM D-l5 4 l-68 7Standard Test for Color of Transparent Liquids (Gardner Scale) is usedto describe the color of liquids that are transparent but relativelydark; the platinum-cobalt color scale is used to describe liquidsthatare very light in color, that is, close to "water-White" I Signed andsealed this 25th day of December 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. D. TEGTMETZER f Attesting Officer ActingCommissioner of l atents ORM PC4050 (10-69)

2. The process of claim 1 in which the alkali metal borohydride issodium borohydride.
 3. The process of claim 1 in which the alkali metalborohydride is lithium borohydride.
 4. The process of claim 1 in whichthe temperature is between 45* and 55* C.
 5. The process of claim 1 inwhich the processing period is between 15 minutes and 1 hour.
 6. Theprocess of claim 1 in which the alkali metal borohydride is present inan amount of 0.01 to 0.02 percent by weight based on the phosphateester.